In a cable television system, broadcast video, video-on-demand, telephony and Internet access are presently delivered over coaxial cable. However, there is a need to deliver these services over optical fiber to multiple dwelling units (MDUs), such as apartment buildings/flats, condominiums, row houses, dorms etc. and to detached dwellings in fiber-to-the-curb or fiber-to-the-cabinet (FTTC/FTTcab) configurations. This need is principally due to certain costs and capacity limitations in the Data over Cable Service Interface Specification (DOCSIS) technology, which presently is used to deliver Internet access to subscribers over cable.
A particular problem in fiber-to-the-building for MDU (FTTB-MDU), FTTC/FTTcab configurations is support of return traffic from existing subscriber communications units (SCU), such as set-top boxes, where the existing SCUs may not be optimized for use in the FTB-MDU and FTTC/FTTcab configurations.
Broadcast television and on-demand video is presently delivered to subscribers over coaxial cable. The SCUs mediate subscriber access to video services, such as premium channels, pay-per-view and electronic program guides. Both digital and analog SCUs use the return path for interactive services (such as pay-per-view) and for status monitoring. In existing cable television systems, the return path is a frequency band (e.g., from 7 MHz to 42 MHz) which is configured to be transported from the SCUs to the headend (HE). Cable operators have a very large investment in SCUs and corresponding headend equipment, and need to use them without modification in FTTB-MDU and FTTC/FTTcab configurations. Attempts have been made and are continuing to be made in an attempt to support SCU return in FTTB-MDU and FTTC/FTTcab systems. That is, to support legacy cable system SCUs in a packet based system.
In one particular system, a digital return is implemented for the entire upstream band, as is presently done in optical nodes in existing cable television systems. That is, the entire upstream band is sampled (i.e., at least at the Nyquist rate) and transported as a stream of digitized samples. However, the wideband requirement of such a system requires high-speed digital transmission, typically over a separate fiber. Required wideband analog-to-digital converters (ADC) at a high cost also are required. Further, such an approach is wasteful, since SCUs transmit infrequently and do so over a relatively small portion of the spectrum. As a variant of this approach, a smaller portion of the upstream band could be digitized, though at the loss of generality and network flexibility, and with little gain in efficiency.
In an alternate system, a digital return is implemented for a single upstream channel. This requires all SCUs served by the FTTB-MDU or FTTC node (but not necessarily every SCU in the system) to be configured by the operator to use the same upstream channel, without frequency hopping. This also requires that all systems work with the same protocol exclusively. Thus, system level flexibility and generality is lost. Again, there remains inefficient use of the return channel due to transmission of sampled data when no signal is present.
In another system, the return demodulation is performed locally and encapsulates the recovered data packets in standardized data packets (e.g., IP datagrams or Ethernet frames), as done in existing headends. This “packetized” data is then sent upstream via the packet network where the resulting requests are processed at the headend. Unfortunately, a different demodulator is needed for each system supported. In addition, the solution at the headend is highly dependent upon an ability to forward recovered data packets, possibly encapsulated in IP or Ethernet in some proprietary way, to proprietary interactive TV controller systems.
In still another system, the entire 7-42 MHz band is amplitude modulated and transmitted as an optical signal over an optical fiber toward the headend. Unfortunately, a second optical fiber, an analog optical transmitter and an analog optical receiver for each FTTH-MDU or FTTC unit is required. Further, analog optical components are significantly more expensive than digital optical components, thereby increasing the cost.